A Passive Optical Network, PON, is a point-to-multipoint network architecture employing fibre cables from a central office to premises. It employs unpowered optical splitters to enable a single optical fibre to serve multiple premises. A PON comprises an Optical Line Terminal, OLT, at the central office of the service provider. It comprises a number of Optical Network Terminals, ONTs, near end users. A PON configuration reduces the amount of fiber and central office equipment required compared with point-to-point architectures. A passive optical network is a form of fiber-optic access network.
In order to supervise and monitor the performance of a PON, Optical Time-Domain Reflectometry, OTDR, is used. An OTDR device injects a series of optical pulses into the fiber. The series of optical pulses, also called OTDR signal travel down the network towards the ONTs. Parts of the OTDR signals are reflected back towards the OTDR device. The back reflected, or backscattered, OTDR signal may be used for estimating the fiber's length and overall attenuation, including splitter losses. The backscattered OTDR signal may also be used to locate faults, such as breaks, and to measure optical return loss.
However, the amount of light scattered back to the OTDR for measurement is quite small, about one-millionth of the optical power in the test pulse. Since so little of the light comes back to the OTDR for analysis, the OTDR receiver circuit must be very sensitive. That means that big reflections, which may be one percent of the outgoing signal, will saturate the receiver, or overload it.
Another difficulty with OTDR measurements is that backscattered light from the fibers between the splitter and the ONTs will be summarized passing back through the splitter and again attenuated, making it very difficult to obtain the magnitude of a possible fault in one or more fibres between the splitter and the ONTs.
As a consequence, misinterpretations may arise when reading or analyzing the backscattered OTDR signal, also referred to as the trace, after the splitter. Even if a fault is detected after the splitter, i.e. between the splitter and the ONTs, it is impossible to define which drop-branch, i.e. which fiber between the splitter and the ONTs, is affected as the received backscattered signal is a superposition of power coming back from all of the drop links.